The discovery of metal-like electrical properties of polyacetylene when exposed to oxidizing agents like iodine vapor in 1977 by Shirakawa, Heeger and MacDiarmid introduced of a group of new members into the semiconductor family—the organic semiconductors and earned the discoverers the Nobel Prize for chemistry in the year 2000. This discovery created a tremendous opportunity as the organic semiconductors exhibit a combination of properties of metals and plastics, being conductive and flexible at the same time. Since then, efforts of the industry and research groups have been made on synthesis of new organic semiconductors and on studying their properties for different applications.
Various electronic and optoelectronic devices have been developed using small molecules organic semiconductors or conducting polymers. Some examples are: organic light emitting devices (OLEDs), organic thin film transistors (OTFTs) and organic solar cells. Compared to devices and circuits fabricated using inorganic semiconductors such as silicon (Si) or gallium arsenide (GaAs), the organic devices and circuits have the advantages of low fabrication cost, large substrate area and flexibility. Possible applications of the organic devices include: light sources, electronic displays, circuits, photovoltaic energy conversion and optical signal detection.
One of the most important electronic properties of the organic semiconductors is the electrical resistivity (or conductivity) of the materials. Many factors govern the electrical resistivity of an organic material, such as polymeric structure, molecular size and impurities. The electrical resistivity also depends on whether or not and how much a dopant is introduced in the material.
Usually, device engineers need to know beforehand the properties of the semiconductor materials in order to design and construct devices like light emitting diodes (LEDs) and solar cells with superior performances. Semiconducting material producers therefore are required to supply their customers with the resistivity of their semiconducting materials. On the other hand, the obtained resistivity results will help material scientists to adjust their synthesis process to obtain organic semiconducting materials with electrical resistivity (or conductivity) in the desired ranges. Unfortunately, the electrical conductivity values reported by material manufactures vary widely from the actual values, caused by lack of a standardized resistivity measurement method. These added uncertainties will make it more difficult for the device engineers to design and simulate device structures.
Conducting organic materials supplied by manufactures today are normally in forms of chunks (solid form). In the case of conducting polymers, they sometimes are dissolved in popular solvent (liquid form). It is almost impossible to determine the resistivity of an organic semiconductor in chunks. For polymers in liquid form, because most of undoped polymers are not good conductors, it is rather difficult to accurately measure the resistivity of polymers in a liquid. Furthermore, the resistivity values of polymers quoted at different concentration and dissolved in different solvents also add confusion in the already vague specification sheet of conducting polymers.
The traditional four-point probe measurement method has been used routinely for the evaluation of resistivity of thin films semiconductors in the inorganic semiconductor industry. This method is accurate and easy to use. However, due to the large resistivity of undoped organic semiconducting materials and high contact resistance presenting between the metal probes and the thin organic films, the current that can pass through an organic semiconducting film through the needle probes is extremely small and not easy to measure, hence, the traditional four-point probe method ceases to be an effective way of evaluating the resistivity of an organic semiconducting film.
From the above-stated comments, it is clear that a structure and a method for standardized resistivity measurements are in need for semiconductors in general and for organic semiconductors in particular. This method should be accurate and easy to use and should provide an effective and standardized tool for both material scientists and device engineers.